Optical recording medium and process for producing the same, and data recording method and data reproducing method for optical recording medium

ABSTRACT

An optical recording medium  10  of the present invention has a support substrate  11  and a light-transmitting layer  12 , and further has a first dielectric layer  31 , a noble metal nitride layer  23 , a second dielectric layer  32 , a light absorption layer  22 , a third dielectric layer  33 , and a reflection layer  21 , all of which are interposed between the light-transmitting layer  12  and the support substrate  11 . In the optical recording medium of the present invention, a laser beam  40  is irradiated on the substrate from the light entrance face  12   a , to thus locally decompose the noble metal nitride layer  23 , so that record marks can be formed by means of resultant bubble pits. In this case, a gas filling the bubble pits, which are to form the record marks, is a chemically-stable nitrogen gas (N 2 ). The risk of this gas oxidizing or corroding other layers is very remote, and high storage reliability can be achieved.

FIELD OF THE INVENTION

The present invention relates to an optical recording medium and amethod for manufacturing the optical recording medium, and moreparticularly, to an optical recording medium of a type in which recordmarks are formed by means of generation of a gas, as well as to a methodfor manufacturing the recording medium. Moreover, the present inventionrelates to a method for recording and reproducing data on and from anoptical recording medium, as well as to a method for recording andreproducing data on and from an optical recording medium of a type inwhich record marks are formed by generation of a gas.

BACKGROUND

In recent years, optical recording mediums typified by a CD (CompactDisc) and a DVD (Digital Versatile Disc) have been widely used asrecording mediums used for recording a large volume of digital data.

Among CDs, a CD of a type (CD-ROM) that does not allow additionalwriting or rewriting of data has a structure where a reflection layerand a protective layer are stacked on a light-transmitting substratehaving a thickness of about 1.2 mm. Reproduction of data can be carriedout by irradiating a laser beam having a wavelength of about 780 nm ontoa reflection layer from the light-transmitting substrate side.Meanwhile, a CD of a type (a CD-R) that enables additional writing ofdata and a CD of a type (a CD-RW) that enables rewriting of data have astructure where a recording layer is added between thelight-transmitting substrate and the reflection layer. Recording andreproduction of data can be carried out by irradiating a laser beamhaving a wavelength of about 780 nm onto a recording layer from thelight-transmitting substrate side.

In the field of CDs, an objective lens having a numerical aperture ofabout 0.45 is used for focusing purpose, whereby a spot of a laser beamon the reflection layer or the recording layer is narrowed to a size ofabout 1.6 μm. Thus, in the field of CDs, a recording capacity of about700 MB and a data transfer rate of about 1 Mbps are achieved at astandard linear velocity (about 1.2 m/sec.).

Of DVDs, a DVD of a type (a DVD-ROM) that does not allow additionalwriting or rewriting of data has a structure formed by bonding togethera laminated body, which is formed by stacking a reflection layer and aprotective layer on a light-transmitting substrate having a thickness ofabout 0.6 mm, and a dummy substrate having a thickness of about 0.6 mm,by way of an adhesive layer. Data can be reproduced by irradiating alaser beam having a wavelength of about 635 nm onto the reflection layerfrom the light-transmitting substrate side. Meanwhile, a DVD of a type(a DVD-R or the like) that enables additional writing of data and a DVDof a type (a DVD-RW or the like) that enables rewriting of data have astructure where a recording layer is additionally interposed between thelight-transmitting substrate and the reflection layer. Recording andreproduction of data can be carried out by irradiating on the recordinglayer a laser beam having a wavelength of about 635 nm from thelight-transmitting substrate side.

In the field of DVDs, an objective lens having a numerical aperture ofabout 0.6 is used for focusing purpose, whereby a spot of a laser beamon the reflection layer or the recording layer is narrowed to a size ofabout 0.93 nm. As mentioned above, a laser beam, whose wavelength isshorter than that of the wavelength used for CDs, and the objectivelens, which is greater in numerical aperture than that used for CDs, areused for recording and reproducing data on and from a DVD. Therefore, abeam spot is smaller than that achieved in the field of CDs is realized.On this account, in the field of DVDs, a recording capacity of about 4.7GB/surface and a data transfer rate of about 11 Mbps at a standardlinear velocity (of about 3.5 m/sec.) are fulfilled.

An optical recording medium, which has a data recording capacity inexcess of that of a DVD and can fulfill a data transfer rate in excessof that of a DVD, has recently been put forth. In relation to such anext-generation-type optical recording medium, a laser beam having awavelength of about 405 nm and an objective lens having a numericalaperture of about 0.85 are used for achieving a large capacity and ahigh data transfer rate. On this account, the spot of the laser beam isnarrowed to a size of about 0.43 μm, and a recording capacity of about25 GB/surface and a data transfer rate of about 36 Mbps at a standardlinear velocity (of about 4.9 m/sec.) can be realized.

As mentioned above, since the objective lens having a very highnumerical aperture is used for the next-generation type opticalrecording medium, the thickness of a light-transmitting layer, which isto become an optical path for the laser beam, is set to an extremely lowthickness on the order of about 100 μm, in order to ensure a sufficienttilt margin and prevent occurrence of coma aberration. For this reason,the optical recording medium of the next-generation type encountersdifficulty in forming various types of functional layers, such as therecording layer, on the light-transmitting substrate as in the case of aCD, a DVD, an optical recording medium of a current type, and the like.Currently under review is a method for forming a reflection layer and arecording layer, in the form of films, on a support substrate; forming athin resin layer on these layers by means of the spin coating method, orthe like; and using the thus-formed resin layer as a light-transmittinglayer. In contrast with the optical recording medium of the current typefor which films are formed in sequence from a light entrance face, filmsare formed in reverse sequence from the light entrance face duringmanufacture of the optical recording medium of the next-generation type.

As has been described, an increase in the capacity and data transferrate of the optical recording medium have been achieved chiefly by areduction in the size of the laser beam spot. Consequently, in order toachieve a further increase in the capacity and data transfer rate, thereis a necessity for decreasing the size of the beam spot. However, if thewavelength of the laser beam is shortened further, absorption of thelaser beam in the light-transmitting layer will abruptly increase, orage deterioration of the light-transmitting layer will become greater.For these reasons, further shortening of the wavelength is difficult.Additionally, in consideration of difficulty in the design of a lens,assurance of a tilt margin, or the like, a further increase in thenumerical aperture of the objective lens is also difficult. In short, afurther reduction in the size of the laser beam spot can be said to beextremely difficult.

With this being the case, an optical recording medium ofsuper-resolution type has recently been put forward as another attemptto achieve increased capacity and a higher data transfer rate. Theoptical recording medium of super-resolution type refers to an opticalrecording medium which enables formation of minute record marks, passinga resolution limit, and reproduction of data from these record marks. Ifsuch an optical recording medium is used, a greater capacity and ahigher data transfer rate can be fulfilled without involvement of areduction in the size of the beam spot.

By way of more detailed explanation, provided that the wavelength of thelaser beam is taken as λ and the numerical aperture of the objectivelens is taken as NA, a diffraction limit d₁ is given byd ₁=λ/2NAIn an optical recording medium of a type, such as a CD or a DVD, inwhich data are expressed by the length of a record marks and the lengthof a blank region; namely, a distance between edges, a resolution limitd₂ of a single frequency signal is given byd ₂=λ/4NASpecifically, in an ordinary optical recording medium which is not of ansuper-resolution type, if the length of the shortest record marks or thelength of the shortest blank region falls below the resolution limit,discriminating between the record marks and the blank region becomesdifficult. In contrast, in the optical recording medium of thesuper-resolution type, the record marks or the blank region, whoselength falls below the resolution limit, can be utilized. Hence, anincreased capacity and a higher data transfer rate can be fulfilledwithout involvement of a reduction in the size of the beam spot.

An optical recording medium of super-resolution type called a“Scattering-type Super Lens (Super RENS)” (Super Resolution Near-fieldStructure) has hitherto been proposed as the optical recording medium ofsuper-resolution type (see Non-Patent Document 1). A reproduction layerconsisting of a phase-change material layer and a metallic oxide is usedfor this optical recording medium. It is considered that, when exposedto a laser beam, the metallic oxide constituting the reproduction layeris decomposed in a high-energy field in the center of the beam spot andthat metallic fine particles resulting from decomposition of themetallic oxide scatter the laser beam, to thus induce near-field light.The explanation for attaining super-resolution is that, since thenear-field light is consequently irradiated locally onto thephase-change material layer, super-resolution recording andsuper-resolution reproduction can be performed by utilization of a phasechange. When the laser beam has moved away, metal and oxygen, which havebeen produced from decomposition of the reproduction layer, are againbound to thus return to the original metallic oxide. Therefore, repeatedrewriting is said to be feasible.

However, the research conducted by the present inventors has revealedthat the phase change in the phase-change material layer hardly appearsas a signal in the optical recording medium of super-resolution typecalled the “Scattering-type Super RENS,” and that decomposition of thereproduction layer is irreversible. Namely, it has become obvious thatthe optical recording medium of super-resolution type called the“Scattering-type Super RENS” is not a rewritable optical recordingmedium which enables formation of reversible record marks in thephase-change material layer and that the optical recording medium ofsuper-resolution type can be embodied as a write-once optical recordingmedium which enables formation of irreversible record marks in areproduction layer (a noble metal oxide layer) (see Non-Patent Document2).

The reason why minute record marks, which fall below the resolutionlimit, can be formed in the noble metal oxide layer is that a noblemetal oxide layer is locally decomposed in a high energy field in thecenter of the beam spot and that the decomposed area becomes plasticallydeformed by resultant bubble pits. The plastically-deformed area is usedas a record marks, and a non-plastically-deformed area is used as ablank region. However, the reason why data can be reproduced from thethus-formed minute record marks has not yet been elucidated.

[Non-Patent Document 1] “A near-field recording and readout technologyusing a metallic probe in an optical disk,” Jap. J. Appl. Phys., theJapan Society of Applied Physics, 2000, Volume 39, pp. 980 to 981.

[Non-Patent Document 2] “Rigid bubble pit formation and huge signalenhancement in super-resolution near-field structure disk withplatinum-oxide layer,” Applied Physics Letters, American Institute ofPhysics, Dec. 16, 2002, Volume 81, Number 25, pp. 4697-4699.

As mentioned above, in the currently-proposed optical recording mediumsof super-resolution type, an area is locally, plastically deformed by anoxygen gas (O₂) produced from decomposition of the noble metal oxidelayer, and the thus-deformed area is utilized as a record marks.Consequently, a gas which is of higher chemical stability is consideredto be desirable as the gas to be generated. This also applies, in a likemanner, to an optical recording medium which is not an optical recordingmedium of super-resolution type.

Application of the super-resolution technique to the optical recordingmedium is intended for achieving a greater capacity and a higher datatransfer rate. Accordingly, it is considered desirable that data arerecorded and reproduced by use of a laser beam having a shorterwavelength and an objective lens having a greater numerical aperture.

DISCLOSURE OF THE INVENTION

Therefore, the object of the present invention is to provide an opticalrecording medium which enables formation of record marks by means ofgeneration of a chemically-stable gas, and particularly, an opticalrecording medium of super-resolution type and a method for manufacturingthe same.

Another object of the present invention is to provide a method forrecording and reproducing data on and from an optical recording mediumof a type in which record marks are formed by means of generation of achemically-stable gas, through use of a laser beam of shorter wavelengthand an objective lens having a greater numerical aperture.

An optical recording medium of the present invention is characterized bycomprising a substrate; and a noble metal nitride layer provided on thesubstrate. In the optical recording medium of the present invention, alaser beam is irradiated on the substrate from a light entrance face, tothus locally decompose the noble metal nitride layer, so that recordmarks can be formed by means of resultant bubble pits. In this case, agas filling the bubble pits, which are to form the record marks, is achemically-stable nitrogen gas (N₂). The risk of this gas oxidizing orcorroding other layers is very remote, and high storage reliability canbe achieved.

Here, the optical recording medium preferably further includes a firstdielectric layer provided on a light entrance face side of the substratewhen viewed from the noble metal nitride layer; and a second dielectriclayer provided on a side of the substrate opposite the light entranceface thereof when viewed from the noble metal nitride layer. So long asthe noble metal nitride layer is sandwiched between the first and seconddielectric layers, the nitrogen gas (N₂) produced from decomposition ofthe noble metal nitride layer can be stably sealed for a long period oftime, so that higher storage reliability can be acquired.

The optical recording medium preferably further includes a lightabsorption layer and a third dielectric layer, which are provided on aside of the substrate opposite the light entrance face thereof whenviewed from the second dielectric layer and arranged in this sequencewhen viewed from the second dielectric layer. By means of such astructure, the energy of the laser beam irradiated during recordingoperation is efficiently converted into heat, so that an excellentrecording characteristic can be acquired.

Moreover, the optical recording medium preferably further includes areflection layer provided on a side of the substrate opposite the lightentrance face thereof when viewed from the third dielectric layer. Solong as such a reflection layer is provided, the intensity of areproduced signal is enhanced, and reproduction stability issignificantly improved. Now, the term “reproduction stability” meansresistance to a reproduction deterioration phenomenon; namely, aphenomenon in which the state of the noble metal nitride layer ischanged by the energy of the laser beam irradiated during reproducingoperation to thus cause an increase in noise and a drop in the number ofcarriers, thereby decreasing a CNR. The thickness of the reflectionlayer preferably ranges from 5 nm to 200 nm, more preferably from 10 nmto 150 nm. So long as the thickness of the reflection layer is set asdescribed above, an advantage of a sufficient improvement inreproduction stability can be yielded without involvement of a greatdrop in productivity.

The noble metal nitride layer preferably contains platinum nitride(PtNx). In this case, most preferably, substantially the entire noblemetal nitride layer is formed from platinum nitride (PtNx). However,other materials or impurities which are inevitably mixed may becontained in the noble metal nitride layer. So long as platinum nitride(PtNx) is used as the material of the noble metal nitride layer, asuperior signal characteristic and sufficient stability can be acquired.

The optical recording medium preferably further includes alight-transmitting layer which is provided opposite to the substratewhen viewed from the first dielectric layer and has the light entranceface. In this case, the thickness of the substrate preferably rangesfrom 0.6 mm to 2.0 mm; the thickness of the light-transmitting layerpreferably ranges from 10 μm to 200 μm; the thickness of the noble metalnitride layer preferably ranges from 2 nm to 75 nm; the thickness of thesecond dielectric layer preferably ranges from 5 nm to 100 nm; thethickness of the light absorption layer preferably ranges from 5 nm to100 nm; and the thickness of the third dielectric layer preferablyranges from 10 nm to 140 nm. By means of this structure,super-resolution recording and reproduction can be performed by means ofsetting λ/NA to 640 nm or less by use of a laser beam having awavelength (λ) of less than about 635 nm and an objective lens having anumerical aperture (NA) of about 0.6-plus. Particularly, a superiorcharacteristic can be acquired through super-resolution recording andsuper-resolution reproduction involving use of a laser beam, whosewavelength is about 405 nm and which is to be used with an opticalrecording medium of next generation type, and an objective lens having anumerical aperture of about 0.85.

A method for manufacturing an optical recording medium of the presentinvention is characterized by comprising a first step of forming on asupport substrate, in this sequence, a reflection layer, a thirddielectric layer, a light absorption layer, a second dielectric layer, anoble metal nitride layer, and a first dielectric layer; and a secondstep of forming a light-transmitting layer on the first dielectriclayer. According to the present invention, there can be formed anoptical recording medium which enables super-resolution recording andreproduction by means of setting λ/NA to 640 nm or less by use of alaser beam having a wavelength of less than about 635 nm and anobjective lens having a numerical aperture of about 0.6-plus. Moreover,in the thus-manufactured optical recording medium, bubble pits formedfrom a chemically-stable nitrogen gas (N₂) are to form record marks.Hence, high storage reliability can be achieved. Processing pertainingto the first step is preferably performed by means of a vapor phasedeposition method, and processing pertaining to the second step ispreferably performed by means of a spin coating method.

A data recording method of the present invention is a data recordingmethod for recording data on the previously-described optical recordingmedium, to thus record data by irradiating a laser beam from the lightentrance face, characterized in that, when a wavelength of the laserbeam is taken as λ and a numerical aperture of an objective lens usedfor focusing the laser beam is taken as NA, a train of record marks,including record marks whose lengths are λ/4NA or less, is recorded bysetting λ/NA to 640 nm or less. Moreover, a data reproduction method ofthe present invention is a data reproduction method for reproducing datafrom the optical recording medium, to thus record data by irradiating alaser beam on the above-described optical recording medium from thelight entrance face, characterized in that, when a wavelength of thelaser beam is taken as λ and a numerical aperture of an objective lensused for focusing the laser beam is taken as NA, data are reproducedfrom a train of record marks, including record marks whose lengths areλ/4NA or less, by setting λ/NA to 640 nm or less. In any case, settingthe wavelength of the laser beam to about 405 nm and setting thenumerical aperture of the objective lens to about 0.85 is mostpreferable. By means of these settings, a recording/reproducing deviceanalogous to a recording/reproducing device for use with an opticalrecording medium of next generation type can be used, and hence cost fordeveloping and manufacturing the recording/reproducing device can becurtailed.

As mentioned above, the optical recording medium of the presentinvention has a noble metal nitride layer provided on a substrate, andbubble pits stemming from decomposition of the noble metal nitride layerare utilized as record marks. To do this, a gas filling the bubble pits,which are to become record marks, is a chemically-stable nitrogen gas(N₂). Consequently, the risk of the nitrogen gas (N₂) filling the bubblepits oxidizing or corroding other layers of the substrate, or the like,is very remote, and high storage reliability can be attained.

Moreover, the optical recording medium of the present invention enablessuper-resolution recording and reproduction by means of setting λ/NA to640 nm or less by use of a laser beam having a wavelength of less thanabout 635 nm and an objective lens having a numerical aperture of about0.6-plus. Particularly, a superior characteristic can be acquiredthrough super-resolution recording and super-resolution reproductioninvolving use of a laser beam, whose wavelength is about 405 nm andwhich is to be used with an optical recording medium of next generationtype, and an objective lens having a numerical aperture of about 0.85.Consequently, a recording/reproducing device analogous to arecording/reproducing device for use with an optical recording medium ofnext generation type can be used, and hence cost for developing andmanufacturing the recording/reproducing device can be curtailed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) is a cutaway perspective view showing the appearance of anoptical recording medium 10 of a preferred embodiment of the presentinvention, and FIG. 11(b) is an enlarged fragmentary cross-sectionalview of section A shown in FIG. 1(a);

FIG. 2 is a view diagrammatically showing a state where the opticalrecording medium 10 is exposed to a laser beam 40;

FIG. 3(a) is a plan view showing a beam spot of the laser beam 40 on anoble metal nitride layer 23, and FIG. 3(b) is a view showing theintensity distribution of the beam spot;

FIG. 4 is a view for describing the size of a bubble pit 23 a (a recordmarks);

FIG. 5 is a waveform diagram showing an example intensity modulationpattern of the laser beam 40 achieved during recording operation;

FIG. 6 is a waveform diagram showing another example intensitymodulation pattern of the laser beam 40 achieved during recordingoperation;

FIG. 7 is a graph diagrammatically showing a relationship between therecording power of the laser beam 40 and a CNR of a reproduced signalformed through subsequent reproduction;

FIG. 8 is a graph diagrammatically showing a relationship between thereproducing power of the laser beam 40 and the CNR;

FIG. 9 is a graph showing measurement results acquired in Evaluation 1of characteristics;

FIG. 10 is a graph showing measurement results acquired in Evaluation 2of characteristics; and

FIG. 11 is a graph showing measurement results acquired in Evaluation 3of characteristics.

BEST MODE FOR IMPLEMENTING THE INVENTION

Preferred embodiments of the present invention will be described indetail hereunder by reference to the accompanying drawings.

FIG. 1(a) is a cutaway perspective view showing the appearance of anoptical recording medium 10 of a preferred embodiment of the presentinvention, and FIG. 1(b) is an enlarged fragmentary cross-sectional viewof section A shown in FIG. 1(a).

As shown in FIG. 1(a), the optical recording medium 10 of the presentembodiment assumes the shape of a disc. As shown in FIG. 1(b), theoptical recording medium comprises a support substrate 11; alight-transmitting layer 12; a reflection layer 21, a light absorptionlayer 22, and a noble metal nitride layer 23, which are interposed inthis sequence between the support substrate 11 and thelight-transmitting layer 12; a dielectric layer 31 interposed betweenthe noble metal nitride layer 23 and the light-transmitting layer 12; adielectric layer 32 interposed between the light absorption layer 22 andthe noble metal nitride layer 23; and a dielectric layer 33 interposedbetween the reflection layer 21 and the light absorption layer 22.Recording and reproduction of data can be performed by irradiating alaser beam 40 on a light entrance face 12 a while the optical recordingmedium 10 is being rotated. The wavelength of the laser beam 40 can beset to a value of less than 635 nm. Especially, it is most preferable toset the wavelength to a wavelength of about 405 nm used for an opticalrecording medium of next-generation type. Moreover, a numerical apertureof an objective lens used for focusing the laser beam 40 can be set to avalue of 0.6-plus. Particularly, the numerical aperture can be set to anumerical aperture of about 0.85 used for an optical recording medium ofnext-generation type.

The support substrate 11 is a disc-shaped substrate used for ensuringmechanical strength required by the optical recording medium 10. Agroove 11 a and a land 11 b, which are used for guiding the laser beam40, are helically formed in one surface of the support substrate fromthe neighborhood of the center to an outer rim of the substrate, or fromthe outer rim to the neighborhood of the center. No specific limitationsare imposed on the material and thickness of the support substrate 11,so long as sufficient mechanical strength can be ensured. For example,glass, ceramic, resin, and the like can be used as the material of thesupport substrate 11. In consideration of ease of molding, resin isdesirably used. Examples of such resin include polycarbonate resin,olefin resin, acrylic resin, epoxy resin, polystyrene resin,polyethylene resin, polypropylene resin, silicone resin, fluorine-basedresin, ABS resin, and urethane resin. Among these resins, polycarbonateresin and olefin resin are especially preferred, in view of ease ofprocessing. Since the support substrate 11 does not become an opticalpath for the laser beam 40, there is no necessity for selecting amaterial which exhibits a high light transmission characteristic in thewavelength range.

The thickness of the support substrate 11 is preferably set to athickness which is required and sufficient to ensure mechanicalstrength; e.g., a range of 0.6 mm to 2.0 mm. In consideration ofinterchangeability between the current optical recording medium and thenext-generation-type optical recording medium, the thickness ispreferably set to a range of 1.0 mm to 1.2 mm, particularly preferably avalue of 1.1 mm or thereabouts. Although no specific limitations areimposed on the diameter of the support substrate 11, the diameter ispreferably set to about 120 mm, in consideration of interchangeabilitybetween the current optical recording medium and thenext-generation-type optical recording medium.

The light-transmitting layer 12 is a layer which is to become an opticalpath for the laser beam 40 irradiated during recording and reproducingoperations. No specific limitations are imposed on the material of thelight-transmitting layer 12, so long as the material exhibits asufficiently-high light transmittance in the wavelength range of theemployed laser beam 40, and, for instance, a light-transmitting resin orthe like can be used. In the optical recording medium 10 of the presentembodiment, the thickness of the light-transmitting layer 12 is set to arange of 10 μm to 200 μm. The reason for this is that, when thethickness of the light-transmitting layer 12 is less than 10 μm, thebeam size on the light entrance face 12 a becomes extremely small, andas a result, flaws or dust on the light entrance face 12 a exertexcessive influence on recording and reproduction. When the thickness is200 μm or higher, difficulty is encountered in assuring a tilt margin orsuppressing coma aberration. In consideration of interchangeabilitybetween the current optical recording medium and thenext-generation-type optical recording medium, the thickness ispreferably set to a range of 50 μm to 150 μm, particularly preferably arange of 70 μm to 120 μm.

The reflection layer 21 plays a role in enhancing the intensity of areproduced signal and reproduction stability. A single metal, such asgold (Au), silver (Ag), copper (Cu), platinum (Pt), aluminum (Al),titanium (Ti), chromium (Cr), iron (Fe), cobalt (Co), nickel (Ni),magnesium (Mg), zinc (Zn), or germanium (Ge), or an alloy thereof can beused as the material of the reflection layer 21. No specific limitationsare imposed on the thickness of the reflection layer 21. However, thethickness is preferably set to a range of 5 nm to 200 nm, morepreferably a range of 10 nm to 150 nm. This is because a sufficientadvantage of an improvement in reproduction stability cannot be acquiredwhen the thickness of the reflection layer 21 is less than 5 nm. Whenthe thickness of the reflection layer 21 exceeds 200 nm, film growthentails consumption of much time, which in turn deterioratesproductivity, and produces no substantial enhancement of reproductionstability. In contrast, when the thickness of the reflection layer 21 isset to a range of 10 nm to 150 nm, a sufficient advantage ofreproduction stability can be achieved without greatly deterioratingproductivity. In the present invention, providing the reflection layer21 on the optical recording medium is not indispensable. However, whenthe reflection layer is provided, the above-described advantage can beyielded.

The light absorption layer 22 chiefly plays a role in absorbing theenergy of the laser beam 40 and converting the thus-absorbed energy intoheat. A material, which exhibits high absorption in the wavelength rangeof the employed laser beam 40 and has comparatively-low hardness so asnot to prevent deformation of the noble metal nitride layer 23 duringrecording, is preferably used as the light absorption layer. Aphase-change material used as a material for a recording layer in arewritable optical recording medium is mentioned as a material whichsatisfies requirements of the laser beam 40 having a wavelength of lessthan 635 nm. The phase-change material is preferably an alloy ofantimony (Sb), tellurium (Te), or germanium (Ge), or a material formedby addition of additives to one of these alloys.

Specifically, provided that an atomic ratio of phase-change materialsforming the light absorption layer 22 is expressed as(Sb_(a)Te_(1-a))_(1-b)MA_(b) or{(GeTe)_(c)(Sb₂Te₃)_(1-c)}_(d)MB_(1-d)

[where MA denotes elements exclusive of antimony (Sb) and tellurium(Te); and MB denotes elements exclusive of antimony (Sb), tellurium(Te), and germanium (Ge)],

the atomic ratio is preferably set to fall within a range such that0≦a≦1, and 0≦b≦0.25 or1/3≦c≦2/3, and 0.9≦d.

Particularly, when the value of “b” exceeds 0.25, a light absorptioncoefficient may become lower than the intensity required by the lightabsorption layer 22, and thermal conductivity may become lower than thevalue required by the light absorption layer 22, as well. Hence, thevalue of “b” preferably does not exceed 0.25.

Although no specific limitations are not imposed on the type of theelement MA, one element or two or more elements are preferably selectedfrom the group consisting of germanium (Ge), indium (In), silver (Ag),gold (Au), bismuth (Bi), selenium (Se), aluminum (Al), phosphor (P),hydrogen (H), silicon (S), carbon (C), vanadium (V), tungsten (W),tantalum (Ta), zinc (Zn), manganese (Mn), titanium (Ti), tin (Sn),palladium (Pd), lead (Pb), nitrogen (N), oxygen (O), and rare-earthmetals [scandium (Sc), yttrium (Y), and lanthanoids]. Particularly, whena laser beam having a wavelength of 390 nm to 420 nm is used, theelement MA is preferably one element or two or more elements from thegroup consisting of silver (Ag), germanium (Ge), indium (In), andrare-earth elements. As a result, a superior signal characteristic canbe acquired when a laser beam having a wavelength of 390 nm to 420 nm,particularly a laser beam having a wavelength of about 405 nm, is used.

Although no specific limitations are not imposed on the type of theelement MB, one element or two or more elements are preferably selectedfrom the group consisting of indium (In), silver (Ag), gold (Au),bismuth (Bi), selenium (Se), aluminum (Al), phosphor (P), hydrogen (H),silicon (S), carbon (C), vanadium (V), tungsten (W), tantalum (Ta), zinc(Zn), manganese (Mn), titanium (Ti), tin (Sn), palladium (Pd), lead(Pb), nitrogen (N), oxygen (O), and rare-earth metals [scandium (Sc),yttrium (Y), and lanthanoids]. Particularly, when a laser beam having awavelength of 390 nm to 420 nm is used, the element MB is preferably oneelement or two or more elements from the group consisting of silver(Ag), indium (In), and rare-earth elements. As a result, a superiorsignal characteristic can be acquired when a laser beam having awavelength of 390 nm to 420 nm, particularly a laser beam having awavelength of about 405 nm, is used.

Even when a phase-change material is used as the material of the lightabsorption layer 22, a phase change due to recording hardly appears as asignal. This is the reason why using the phase-change material as thematerial of the light absorption layer 22 is not indispensable. However,the inventors have ascertained that the best signal characteristic isacquired when the phase-change material is used as the material of thelight absorption layer 22, particularly a phase-change material whichhas the above-described composition.

When the phase-change material is used as the material of the lightabsorption layer 22, the thickness of the same is preferably set to arange of 50 nm to 100 nm, more preferably a range of 10 nm to 80 nm, andmost preferably a range of 10 nm to 60 nm. The reason for this is that,when the thickness of the light absorption layer 22 is less than 5 nm,sufficient absorption of energy of the laser beam may fail to beattained, and when the thickness of the light absorption layer exceeds100 nm, film growth involves consumption of much time and productivitydrops. In contrast, when the thickness of the light absorption layer 22is set to a range of 10 nm to 80 nm, particularly a range of 10 nm to 60nm, the energy of the laser beam 40 can be sufficiently absorbed whilehigh productivity is ensured.

In the present invention, providing the optical recording medium withthe light absorption layer 22 is not indispensable. However, asmentioned previously, highly-efficient conversion of the energy of thelaser beam 40 into heat can be attained as a result of provision of thelight absorption layer.

The noble metal nitride layer 23 is a layer where a record marks isformed upon exposure to the laser beam 40, and contains a noble metalnitride as the principal constituent. Although no specific limitationsare imposed on the type of noble metal, at least one among platinum(Pt), silver (Ag), and palladium (Pd) is preferable. Platinum (Pt) isparticularly preferable. Specifically, platinum nitride (PtNx) isparticularly preferable as the material of the noble metal nitride layer23. Acquisition of a superior signal characteristic and sufficientstability becomes possible, so long as platinum nitride (PtNx) is usedas the material of the noble metal nitride layer 23. When platinumnitride (PtNx) is used as the material of the noble metal nitride layer23, the value of “x” is preferably set such that an extinctioncoefficient (k) becomes smaller than 3 (k<3) within the wavelength rangeof the employed laser beam 40.

The thickness of the noble metal nitride layer 23 exerts great influenceon the signal characteristic. In order to achieve a superior signalcharacteristic, the thickness of the noble metal nitride layer ispreferably set to a range of 2 nm to 75 nm, more preferably a range of 2nm to 50 nm. Particularly, in order to acquire a superior signalcharacteristic for a signal of the diffraction limit or less, thethickness of the noble metal nitride layer is preferably set to a rangeof 2 nm to 15 nm. In a case where the thickness of the noble metalnitride layer 23 is smaller than 2 nm or in excess of 75 nm, a recordmarks having a superior geometry cannot be formed even when the noblemetal nitride layer is exposed to the laser beam 40, and a sufficientcarrier-to-noise ratio (CNR) may fail to be acquired. In contrast, solong as the thickness of the noble metal nitride layer 23 is set to arange of 2 nm to 15 nm, a superior CNR can be acquired in connectionwith a signal of diffraction limit or less as well as to a signal of adiffraction limit or more.

The dielectric layers 31, 32, and 33 chiefly play a role in physicallyand chemically protecting respective adjacent layers and adjusting anoptical characteristic. Throughout the specification and the claims, thedielectric layers 31, 32, and 33 are often called a first dielectriclayer, a second dielectric layer, and a third dielectric layer,respectively. Oxides, sulfides, nitrides, or combinations thereof areused as the principal constituent of materials for the dielectric layers31, 32, and 33. Specifically, oxides, nitrides, sulfides, or carbides ofaluminum (Al), silicon (Si), cerium (Ce), titanium (Ti), zinc (Zn), ortantalum (Ta), such as Al₂O₃, AlN, ZnO, ZnS, GeN, GeCrN, CeO₂, SiO,SiO₂, Si₃N₄, SiC, La₂O₃, TaO, TiO₂, SiAlON (mixtures of SiO₂, Al₂O₃,Si₃N₄, and AlN), LaSiON (mixtures of La₂O₃, SiO₂, and Si₃N₄, or thelike), or mixtures thereof are preferably used. Particularly, use of amixture consisting of ZnS and SiO₂ is more preferable. In such a case, aratio of ZnS is preferably set to a range of 70 mol percent to 90 molpercent; a ratio of SiO₂ is preferably set to a range of 10 mol percentto 30 mol percent; and a mol ratio of ZnS to SiO₂ is most preferably setto about 80:20.

The dielectric layers 31, 32, and 33 may be formed from the samematerial, or some or all of them may be formed from different materials.Moreover, at least one of the dielectric layers 31, 32, and 33 may beformed from a multilayer structure comprising a plurality of layers.

The thickness of the dielectric layer 33 is preferably set to a range of10 nm to 140 nm, more preferably to a range of 20 nm to 120 nm. Thereason for this is that, when the thickness of the dielectric layer 33is smaller than 101n, there arises a fear of a failure to sufficientlyprotect the light absorption layer 22, and that, when the thickness ofthe dielectric layer 33 exceeds 140 nm, productivity is deteriorated asa result of film growth involving consumption of much time. In contrast,so long as the thickness of the dielectric layer 33 is set to a range of20 nm to 120 nm, the light absorption layer 22 can be effectivelyprotected while high productivity is ensured. However, when the opticalrecording medium is not provided with the light absorption layer 22, thedielectric layer 33 can be omitted.

The thickness of the dielectric layer 32 is preferably set to a range of5 nm to 100 nm, more preferably a range of 20 nm to 100 nm. The reasonfor this is that when the thickness of the dielectric layer 32 issmaller than 5 nm, the dielectric layer 32 may be broken duringdecomposition of the noble metal nitride layer 23, to thus fail toprotect the noble metal nitride layer 23, and when the thickness of thedielectric layer 32 exceeds 100 nm, the noble metal nitride layer 23 mayfail to be sufficiently deformed during recording. In contrast, so longas the thickness of the dielectric layer 32 is set to a range of 20 nmto 100 nm, the dielectric layer does not excessively hinder deformationof the noble metal nitride layer 23, which arises during recordingoperation, while sufficiently protecting the same. Further, thethickness of the dielectric layer 32 also affects the signalcharacteristic acquired during reproduction of data. A high CNR can beacquired, so long as the thickness of the dielectric layer 32 is set toa range of 50 nm to 70 nm, particularly to a value of about 60 nm.

The essential requirement is to set the thickness of the dielectriclayer 31 according to required reflectivity, so long as the noble metalnitride layer 23 can be sufficiently protected. For instance, thethickness of the dielectric layer 31 is preferably set to a range of 30nm to 120 nm, more preferably a range of 50 nm to 100 nm, and mostpreferably a value of about 70 nm. The reason for this is that, when thethickness of the dielectric layer 31 is smaller than 30 nm, there mayarise a case where the noble metal nitride layer 23 cannot besufficiently protected, and, when the thickness of the dielectric layer31 exceeds 120 nm, productivity is deteriorated as a result of filmgrowth involving consumption of much time. In contrast, so long as thethickness of the dielectric layer 31 is set to a range of 50 nm to 100nm, particularly to a value of about 70 nm, the noble metal nitridelayer 23 can be sufficiently protected while high productivity isensured.

The above describes the structure of the optical recording medium 10.

Manufacture of the optical recording medium 10 having such a structureis achieved by initially preparing the support substrate 11; andsequentially forming the reflection layer 21, the dielectric layer 33,the light absorption layer 22, the dielectric layer 32, the noble metalnitride layer 23, the dielectric layer 31, and the light-transmittinglayer 12 on the face of the support substrate 11 where the grooves 11 aand the lands 11 b are formed. Specifically, during manufacture of theoptical recording medium 10, films are sequentially formed from the faceopposite the light entrance face 12 a, as in the case of the opticalrecording medium of next-generation type.

The reflection layer 21, the dielectric layer 33, the light absorptionlayer 22, the dielectric layer 32, the noble metal nitride layer 23, andthe dielectric layer 31 can be formed by vapor phase deposition usingchemical species containing constituent elements thereof, for example,by sputtering or vacuum evaporation. Above all, using sputtering ispreferable. Meanwhile, the light-transmitting layer 12 can be formed byforming a coating from, e.g., an acrylic or epoxy-based ultraviolet cureresin whose viscosity has been adjusted, by means of the spin-coatingmethod, and exposing the coating to ultraviolet irradiation in anitrogen atmosphere to thus cure the coating. Alternatively, thelight-transmitting layer 12 may be formed by, rather than thespin-coating method, from a light-transmitting sheet containinglight-transmitting resin as the principal constituent and variousbonding agents and pressure-sensitive adhesives.

A hard coating layer may be provided on the surface of thelight-transmitting layer 12, thereby protecting the surface thereof. Inthis case, the surface of the hard coating layer forms the lightentrance surface 12 a. For instance, ultraviolet cure resin containingan epoxy acrylate oligomer (bifunctional oligomer), multifunctionalacrylic monomer, monofunctional acrylic monomer, and aphotopolymerization initiator; or oxides, nitrides, sulfides, orcarbides of aluminum (Al), silicon (Si), cerium (Ce), titanium (Ti),zinc (Zn), tantalum (Ta), or mixtures thereof are used as a material forthe hard coating layer. When an ultraviolet cure resin is used as amaterial of the hard coating layer, the ultraviolet cure resin ispreferably formed over the light-transmitting layer 12 by thespin-coating method. When the oxides, nitrides, sulfides, carbides, ormixtures thereof are used, vapor phase deposition using chemical speciesincluding the above-described constituent elements; for example,sputtering or vacuum deposition, can be employed. Among others, use ofsputtering is preferable.

Since the hard coating layer plays a role in preventing infliction offlaws on the light entrance face 12 a, the hard coating layer preferablyhas lubricity as well as hardness.

In order to impart lubricity to the hard coating layer, an effectivemeasure is incorporate a lubricant into a material (e.g., SiO₂) which isto become a base material of the hard coating layer. The lubricant ispreferably a silicon-based lubricant, a fluorine-based lubricant, or afatty-acid-ester-based lubricant. The lubricant content is preferablyset to a range of 0.1 mass percent to 5.0 mass percent.

The method for and principle employed in recording data on the opticalrecording medium 10 of the present embodiment will now be described.

Data are recorded on the optical recording medium 10 by exposing thenoble metal nitride layer 23 to the laser beam 40 having a wavelength ofless than 635 nm, preferably a wavelength of about 405 nm used for anoptical recording medium of next generation type, by way of the lightentrance face 12 a while the optical recording medium 10 is beingrotated. In this case, an objective lens having a numerical aperture of0.6-plus, particularly, an objective lens which is used for an opticalrecording medium of next generation type and has a numerical aperture ofabout 0.85, can be used as the objective lens for focusing the laserbeam 40. Specifically, data can be recorded by use of an optical systemanalogous to the optical system used for the optical recording medium ofnext generation type.

FIG. 2 is an essential cross-sectional view diagrammatically showing thestate of the optical recording medium 10 exposed to the laser beam 40.The cross section of the optical recording medium 10 shown in FIG. 2corresponds to a cross section taken along the groove 11 a and the land11 b.

As shown in FIG. 2, the laser beam 40 having the above-describedwavelength is focused by an objective lens 50 having the above-describednumerical aperture, and is irradiated onto the optical recording medium10, whereupon the noble metal nitride layer 23 is decomposed in thecenter of the beam spot, and bubble pits 23 a filled with a nitrogen gas(N₂) are generated. Fine particles 23 b of raw metal are dispersed inthe bubble pits 23 a. At this time, layers existing around the bubblepits 23 a are plastically deformed by pressure in the bubble pits, andhence the bubble pits 23 a can be used as irreversible record marks. Forexample, when the material of the noble metal nitride layer 23 is aplatinum nitride (PtNx), the platinum nitride (PtNx) decomposes intoplatinum (Pt) and a nitrogen gas (N₂) in the center of the beam spot,whereby platinum (Pt) fine particles are dispersed in the bubble pits 23a. Of the noble metal nitride layer 23, areas where no bubble pits 23 aare generated act as blank regions. Since the nitrogen gas (N₂) producedfrom decomposition has high chemical stability, the chance of thenitrogen gas oxidizing or corroding other layers is very remote, andtherefore high storage reliability can be achieved.

Decomposition of the noble metal nitride layer 23 does not arise in thearea corresponding to the entirety of the beam spot. As mentionedpreviously, decomposition arises solely in the center of the beam spot.Consequently, the thus-generated bubble pits 23 a (record marks) aresmaller than the beam spot, whereby super-resolution recording isfulfilled. The reason why such super-resolution recording can be carriedout is as follows:

FIG. 3(a) is a plan view showing a beam spot of the laser beam 40 on thenoble metal nitride layer 23, and FIG. 3(b) is a view showing theintensity distribution of the beam spot.

As shown in FIG. 3(a), the plane geometry of the beam spot 41 isessentially circular. However, the intensity distribution of the laserbeam 40 in the beam spot 41 is not uniform, but assumes a Gaussiandistribution as shown in FIG. 3(b). Specifically, energy becomes greaterwith increased proximity to the center in the beam spot 41.Consequently, so long as a predetermined threshold value A is set so asto sufficiently exceed 1/e² of the maximum intensity, a diameter W2 of aregion 42—where intensity greater than the threshold-value A isachieved—becomes sufficiently smaller than the diameter W1 of the beamspot 41. This implies that, so long as the noble metal nitride layer 23has a characteristic of being decomposed upon exposure to the laser beam40 having intensity greater than the threshold value A, the bubble pits23 a (record marks) are selectively formed in only an area of the regionexposed to the laser beam 40, the area corresponding to the region 42 inthe beam spot 41.

Thereby, as shown in FIG. 4, the bubble pits 23 a (record marks), whichare sufficiently smaller than the diameter W1 of the beam spot, can beformed in the noble metal nitride layer 23, and the diameter of thebubble pits becomes essentially equal to W2. Specifically, therelationship between the diameter W2 of the apparent beam spot and thediameter W1 of the actual beam spot becomes W1>W2, so thatsuper-resolution recording is fulfilled.

Therefore, so long as the laser beam 40 having modified intensity isirradiated onto the optical recording medium 10 along the groove 1 aand/or land 11 b while the optical recording medium 10 is being rotated,minute record marks, which are smaller than the resolution limit, can beformed in desired locations on the noble metal nitride layer 23.

FIG. 5 is a waveform chart showing an example of an intensity modulationpattern of the laser beam 40 acquired during recording operation. Asshown in FIG. 5, intensity 40 a of the laser beam 40 required duringrecording operation is set to recording power (=Pw) in regions whererecord marks M1, M2, M3, . . . are to be formed, and is set to basepower (=Pb) in regions (blank regions) where no record marks are to beformed. As a result, the bubble pits 23 a are formed throughdecomposition in the regions exposed to the laser beam 40 havingrecording power Pw in the noble metal nitride layer 23. Accordingly, therecord marks M1, M2, M3, . . . , each having a desired length, can beformed. An intensity modulation pattern of the laser beam 40 acquiredduring recording operation is not limited to that shown in FIG. 5. Forinstance, as shown in FIG. 6, the record marks M1, M2, M3, . . . may beformed through use of divided pulse trains.

FIG. 7 is a graph diagrammatically showing a relationship betweenrecording power of the laser beam 40 and the CNR of a reproduced signalobtained through subsequent reproducing operation.

As shown in FIG. 7, provided that the recording power of the laser beam40 is less than Pw1, an effective reproduced signal is not obtained fromthe optical recording medium 10 even when reproducing operation isperformed subsequently. A conceivable reason for this is that, when therecording power of the laser beam 40 is less than Pw1, the noble metalnitride layer 23 is not substantially decomposed. In the region wherethe recording power of the laser beam 40 is Pw1 to less than Pw2 (>Pw1),a higher CNR is acquired through subsequent reproducing operation asrecording power is greater. A conceivable reason for this is thatdecomposition of the noble metal nitride layer 23 has partially arisenin the regions where the recording power of the laser beam 40 is Pw1 toless than Pw2, and that the extent of decomposition becomes greater asthe recording power becomes greater. In the region where the recordingpower of the laser beam 40 is Pw2 or higher, no substantial changesarise in the CNR obtained through subsequent reproducing operation evenwhen the recording power is increased further. A conceivable reason forthis is that, when the recording power of the laser beam 40 is Pw2 orhigher, the noble metal nitride layer 23 is substantially completelydecomposed. In consideration of the above-mentioned reasons, setting therecording power of the laser beam 40 to Pw2 or higher can be said to bepreferable.

The value of Pw2 changes according to the structure (materials orthicknesses of the respective layers, or the like) of the opticalrecording medium 10 or recording requirements (a recording linearvelocity, the wavelength of the laser beam 40, and the like). When therecording linear velocity is 6.0 m/s or thereabouts, the wavelength ofthe laser beam 40 is about 405 nm, and the numerical aperture of theobjective lens 50 is about 0.85, there is achieved7.0 mW≦Pw2≦11.0 mW, andthere is also achievedPw1×1.4≦Pw2≦Pw1×2.0in connection with Pw1.

In actual setting of recording power, the recording power is preferablyset to a value which is higher than Pw2 by 0.3 mW or more, inconsideration of variations in manufacture of the optical recordingmedium 10, fluctuations in the power of the laser beam 40, and the like.This is because no actual damage arises even when the actual recordingpower is higher than Pw2, and a sufficient margin on Pw2 should beensured. However, recording power which is higher than required iswasteful, and hence there is no necessity for setting the recordingpower to a value which is higher than Pw2 by 2.0 mW or more. Asmentioned above, the only requirement can be said to be setting theactual recording power to 7.3 mW (=7.0 mW+0.3 mW) to 13.0 mW (=11.0mW+2.0 mW).

The above-mentioned are the method for and principle employed inrecording data on the optical recording medium 10.

When the thus-recorded data are reproduced, the essential requirement isto irradiate the laser beam 40 whose intensity is fixed to apredetermined intensity (reproducing power=Pr) onto the opticalrecording medium 10 along the groove 11 a and/or land 11 b while theoptical recording medium 10 is being rotated. An electrical signalcorresponding to a train of record marks can be acquired so long as theobtained reflected light is subjected to photoelectric conversion. Thereason why such super-resolution reproduction is feasible is notnecessarily evident. An inferred reason for this is that, when the laserbeam 40 set to reproducing power is irradiated, some type of interactionarises between the laser beam 40 and the fine metal particles 23 bpresent in the bubble pits 23 a, to thus enable super-resolutionreproduction.

FIG. 8 is a graph diagrammatically showing a relationship between thereproducing power of the laser beam 40 and the CNR.

As shown in FIG. 8, when the reproducing power of the laser beam 40 isunder Pr1, an effective reproduced signal is hardly obtained. However,when the reproducing power is set to Pr1 or higher, the CNR rapidlyincreases. When the reproducing power is increased to Pr2 (>Pr1), theCNR becomes saturated. The reason for occurrence of such a phenomenon isnot necessarily evident. An inferred reason for this is that interactionbetween the fine metal particles 23 b and light arises or becomesnoticeable as a result of irradiation of the laser beam 40 set to Pr1 orhigher. Therefore, the reproducing power of the laser beam 40 must beset to Pr1 or higher and, preferably, the reproducing power is set toPr2 or higher.

However, if the reproducing power is set too high, decomposition of thenoble metal nitride layer 23 may arise in the blank regions. When suchdecomposition has arisen, significant reproduction deterioration willarise or data loss may arise in some cases. In consideration of thispoint, the reproducing power of the laser beam 40 is preferably set toPr2 to less than Pw1.

The value of Pw2 changes according to the structure (materials orthicknesses of the respective layers, or the like) of the opticalrecording medium 10 or reproduction requirements (a reproduction linearvelocity, the wavelength of the laser beam 40, and the like). When thereproduction linear velocity is 6.0 m/s or thereabouts, the wavelengthof the laser beam 40 is about 405 nm, and the numerical aperture of theobjective lens 50 is about 0.85, there is achieved1.0 mW≦Pr2≦3.0 mW, andthere is also achievedPr1×1.05≦Pr2≦Pr1×1.6in connection with Pr1.

In actual setting of reproducing power, the reproducing power ispreferably set to a value which is higher than Pr2 by 0.1 mW to 0.3 mW.This is because, when the reproducing power exceeds Pr2, no improvementarises in the CNR even if the reproducing power is increased further,and reproduction deterioration is likely to arise. For this reason, theactual reproducing power should be set to an intensity which is slightlyhigher than Pr2, in order to suppress occurrence of reproductiondeterioration. Under normal conditions, fluctuations in the power of thelaser beam 40 in the range where an output of 1 mW to 3 mW is achievedare less than 0.1 mW. Therefore, even in consideration of variations inthe manufacture of the optical recording medium, or the like, settingreproducing power so as to become higher than Pr2 by 0.1 mW to 0.3 mW isconsidered to be sufficient. Therefore, actual reproducing power can besaid to be set to 1.1 mW (=1.0 mW+0.1 mW) to 3.3 mW(=3.0 mW+0.3 mW).

Reproducing power of the conventional optical recording medium usuallyranges from 0.1 mW to 0.5 mW or thereabouts. Even in the case of anoptical recording medium of next-generation-type having a two-layerrecording face on one side, reproducing power is rarely set in excess ofabout 0.8 mW. In consideration of this fact, the intensity ofreproducing power employed in the present embodiment is understood to beconsiderably higher than that employed for the conventional opticalrecording medium.

In connection with actual recording power, actual reproducing power ispreferably set toPw×0.1≦Pr≦Pw×0.5,

more preferably toPw×0.1≦Pr≦Pw×0.4.From these relationships, the intensity of reproducing power employed inthe present embodiment is considered to be significantly higher thanthat employed for the conventional optical recording medium.

Values which are to be actually set as the recording power and thereproducing power are preferably stored in the optical recording medium10 as “Setting Information.” So long as such setting information isstored in the optical recording medium 10, the setting information isread by an optical recording/reproducing device when the user actuallyrecords or reproduces data, so that the recording power and thereproducing power can be set on the basis of the thus-read settinginformation.

More preferably, the setting information includes information requiredto specify various requirements (the linear velocity or the like) whichare necessary for recording and reproducing data on and from the opticalrecording medium 10, as well as including the recording power and thereproducing power. The setting information may also be informationrecorded in the form of wobbles or pre-pits, or data recorded in thenoble metal nitride layer 23. Moreover, the setting information may alsobe information which is used for indirectly specifying recording powerand reproducing power by designating any of various requirementspreviously stored in the optical recording/reproducing device, as wellas information which directly shows various requirements required torecord and reproduce data.

As has been described, the optical recording medium of the presentembodiment has the noble metal nitride layer 23 and the dielectriclayers 31, 32 with the noble metal nitride layer sandwichedtherebetween. Accordingly, super-resolution recording and reproductioncan be performed with λ/NA being set to 640 nm or less by use of a laserbeam having a wavelength of less than about 635 nm and an objective lenshaving a numerical aperture of about 0.6-plus. Particularly, a superiorcharacteristic can be achieved during super-resolution recording andreproduction which involve use of the laser beam, which has a wavelengthof about 405 nm and is to be used for an optical recording medium ofnext-generation-type, and an objective lens having a numerical apertureof about 0.85. Consequently, a recording/reproducing device analogous tothat for use with an optical recording medium of next-generation-typecan be used. Therefore, cost for development and manufacture of therecording/reproducing device can be reduced. The gas filling the bubblepits 23 a, which are to become record marks, is a chemically-stablenitrogen gas (N₂). Therefore, the risk of the gas oxidizing or corrodingother layers is very remote, and hence high storage reliability can alsobe realized.

Needless to say, the present invention is not limited to theabove-described embodiments and is susceptible to various modificationswithin the scope of the invention described in claims, and themodifications fall within the scope of the present invention.

For instance, the structure of the optical recording medium 10 shown inFIG. 1 is a mere preferred structure of the optical recording medium ofthe present invention, and the structure of the optical recording mediumof the present invention is not limited to that structure. For instance,another noble metal nitride layer may be provided on a portion of theoptical recording medium closer to the support substrate 11 when viewedfrom the light absorption layer 22, or another light absorption layermay be added to a portion of the optical recording medium closer to thelight-transmitting layer 12 when viewed from the noble metal nitridelayer 23. Further, although the optical recording medium 10 shown inFIG. 1 has a structure which is highly compatible with a so-calledoptical recording medium of next-generation-type, the optical recordingmedium can also be provided with a structure which is highly compatiblewith a so-called optical recording medium of DVD type or an opticalrecording medium of CD type.

Moreover, various functional layers, such as the light absorption layer22, the noble metal nitride layer 23, and the like, are provided on bothsides of the support substrate 11, thereby realizing a structure havingrecording faces on both sides. Alternatively, there can be realized astructure having a recording surface of two or more layers on a singleside by stacking various functional layers into two or more layers onone side of the support substrate 11 by way of a transparentintermediate layer.

In the present embodiment, super-resolution recording and reproductionare performed by use of the laser beam having a wavelength of less thanabout 635 nm and an objective lens having a numerical aperture of about0.6-plus. However, using such a laser beam and the objective lens is notindispensable for recording or reproducing data on or from the opticalrecording medium of the present invention. Recording or reproduction maybe performed through use of a laser beam having a wavelength of about635 nm or more and/or an objective lens having a numerical aperture ofabout 0.6 or less. Using the laser beam having a wavelength of less thanabout 635 nm and the objective lens having a numerical aperture of about0.6-plus is preferable, because more fine record marks can be formed.Further, in the present invention, utilizing the record marks and theblank regions, which are of less than the resolution limit, is notindispensable. Data may be recorded and reproduced by use of only therecord marks and the blank regions which are of greater than theresolution limit. In short, performing the super-resolution recordingand reproduction is not indispensable. However, the optical recordingmedium of the present invention intrinsically enables super-resolutionrecording and reproduction. Therefore, it is preferable to achievegreater capacity and a higher data transfer rate by means of utilizingthe record marks and the blank regions, which are of less than theresolution limit.

In the present embodiment, the noble metal nitride layer 23 issandwiched between the dielectric layers 31, 32. However, when excessivedeformation of the marks sections formed through decomposition of thenoble metal nitride layer 23 can be suppressed, one or both of thedielectric layers 31, 32 can be omitted.

EMBODIMENTS

Embodiments of the present invention will be described hereunder.However, the present invention is not limited to the embodiments.

[Manufacture of Samples]

First Embodiment

An optical recording medium sample having the same structure as that ofthe optical recording medium 10 shown in FIG. 1 was manufactured by thefollowing method.

First, the disc-shaped support substrate 11, which has a thickness ofabout 1.1 mm, a diameter of about 120 mm, and the grooves 11 a and thelands 11 b formed in a front face of the substrate, was formed frompolycarbonate by means of injection molding.

Next, the support substrate 11 was set in a sputtering system. In theside of the support substrate where the groove 11 a and the land 11 bare formed, there were sequentially formed the reflection layer 21 whichis formed essentially from platinum (Pt) and has a thickness of about 20nm; the dielectric layer 33 which is formed essentially from a mixtureconsisting of ZnS and SiO₂ (a mol ratio of about 80:20) and has athickness of about 80 nm; the light absorption layer 22 which is formedessentially from Ag_(a)In_(b)Sb_(c)Te_(d) (a=5.9, b=4.4, c=61.1, d=28.6)and has a thickness of about 60 nm; the dielectric layer 32 which isformed essentially from a mixture consisting of ZnS and SiO₂ (a molratio of about 80:20) and has a thickness of about 60 nm; the noblemetal nitride layer 23 which is formed essentially from a platinumnitride (PtNx) and has a thickness of about 2 nm; and the dielectriclayer 31 which is formed essentially from a mixture consisting of ZnSand SiO₂ (a mol ratio of about 80:20) and has a thickness of about 70 nmby means of sputtering.

Here, at the time of formation of the noble metal nitride layer 23,platinum (Pt) was used as a target, and a nitrogen gas (N₂) and an argongas (Ar) were used as a sputtering gas (a flow ratio of 1:1). Internalpressure of a chamber was set to 0.72 Pa, and sputtering power was setto 100 W. An extinction coefficient (k) of the platinum nitride (PtNx),which has been formed under the settings, with respect to light having awavelength of about 405 nm was about 1.74.

The dielectric layer 31 was coated with an acrylic ultraviolet cureresin by means of spin coating, and the coating was exposed toultraviolet irradiation, to thus form the light-transmitting layer 12having a thickness of about 100 μm. Thus, the optical recording mediumsample of the first embodiment was completed.

Second Embodiment

The thickness of the dielectric layer 33 was set to about 100 nm, andthe thickness of the noble metal nitride layer 23 was set to about 4 nm.In other respects, an optical recording medium sample of a secondembodiment was produced in the same manner as was the optical recordingmedium sample of the first embodiment.

[First Evaluation of Characteristic]

First, the optical recording medium sample of the first embodiment andthat of the second embodiment were set in an optical disk evaluationsystem (DDU1000 manufactured by Pulstec Industrial Co., Ltd.). While theoptical recording medium was being rotated at a linear velocity of about6.0 m/s, a laser beam having a wavelength of about 405 nm was irradiatedonto the noble metal nitride layers 23 from the light entrance faces 12a by way of an objective lens having a numerical aperture of about 0.85,to thus record a single frequency signal having a predetermined recordmark length and a blank length. The record mark length and the blanklength were set to various values within a range of 37.5 nm to 320 nm.When the above-described optical system was used, a resolution limitgiven by the following equation is about 120 nm.d ₂=λ/4NA

In relation to the power of the laser beam 40 employed during recordingoperation, recording power (Pw) of the laser beam was set to anintensity (optimal recording power) at which the highest CNR is achievedin any of the optical recording medium samples. The base power (Pb) wasset to substantially 0 mW. A pattern shown in FIG. 5 was used as a pulsepattern for the laser beam 40.

The thus-recorded single frequency signal was reproduced, and a CNR ofthe reproduced signal was measured. The reproducing power (Pr) of thelaser beam 40 was set to an intensity (optimum reproducing power) atwhich the highest CNR is acquired in the respective optical recordingmedium samples. In relation to the optical recording medium sample 1,optimum recording power of 8.5 mW and optimum reproducing power of 2.4mW were obtained. In relation to the optical recording medium sample 2,optimum recording power of 10.0 mW and optimum reproducing power of 2.4mW were obtained.

FIG. 9 shows measurement results of CNRs.

As shown in FIG. 9, in any of the optical recording medium samples ofthe first and second embodiments, high CNRs can be ascertained to havebeen obtained even when the record mark length and the blank length aresmaller than the resolution limit (about 120 nm). For instance, even ina case where the record mark length and the blank length are 80 nm, aCNR of about 41 dB is obtained from the optical recording medium sampleof the first embodiment, and a CNR of about 40 dB is obtained from theoptical recording medium sample of the second embodiment. As a result,it was ascertained that super-resolution recording and super-resolutionreproduction are possible, so long as the optical recording mediumsamples of the first and second embodiments are used.

[Second Evaluation of a Characteristic]

Next, optical recording medium samples of the first and secondembodiments were set in the above-described optical disk evaluationsystem. Under the same conditions as those used for the above-described“First evaluation of a characteristic,” a single frequency signal whoserecord mark length and blank length are 80 nm was recorded. In relationto the recording power (Pw) of the laser beam 40 employed duringrecording operation, recording power was set to various values within arange of 6.0 mW to 10.5 mW The base power (Pb) was set to substantially0 mW. The pattern shown in FIG. 5 was used as a pulse pattern for thelaser beam 40.

The thus-recorded single frequency signals were reproduced, and CNRs ofthe thus-reproduced signals were measured. The reproducing power (Pr) ofthe laser beam 40 was set to 2.4 mW. Measurement results are shown inFIG. 10.

As shown in FIG. 10, in relation to the optical recording medium sampleof the first embodiment, the CNR has become increased in synchronismwith the recording power in the recording power range of less than 8.5mW. However, in a recording power range of 8.5 mW or higher, the CNR issaturated, and no further improvement is found. Namely, for the opticalrecording medium sample of the first embodiment, the following recordingpower was obtained.Pw2=8.5 mW.

In relation to the optical recording medium sample of the secondembodiment, the CNR is increased in synchronism with the recording powerin the recording power range of less than 10.0 mW. However, in arecording power range of 10.0 mW or higher, the CNR is saturated, and nofurther improvement is found. In short, for the optical recording mediumsample of the second embodiment, the following recording power wasobtained.Pw2=10.0 mW

[Third Evaluation of a Characteristic]

Of the single frequency signals recorded in the “Second Evaluation of acharacteristic,” the single frequency signal recorded in the opticalrecording medium sample of the first embodiment with the recording powerset to 8.5 mW and the single frequency signal recorded in the opticalrecording medium sample of the second embodiment with the recordingpower set to 10.0 mW were reproduced through use of various reproducingpowers, and CNRs of the reproduced data was measured. FIG. 11 showsresults of measurement of resultant CNRs.

As shown in FIG. 11, in any of the optical recording medium samples, theCNR achieved in the reproducing power range of less than 2.0 mW is lessthan 10 dB. When the reproducing power achieves 2.0 mW or higher, theCNR abruptly increases. Specifically, in both the optical recordingmedium samples of the first and second embodiments, the requiredreproducing power isPr2=2.0 mW.

INDUSTRIAL APPLICABILITY

The optical recording medium of the present invention has a noble metalnitride layer provided on a substrate, and bubble pits stemming fromdecomposition of the noble metal nitride layer are utilized as recordmarks. To do this, a gas filling the bubble pits, which are to becomerecord marks, is a chemically-stable nitrogen gas (N₂). Consequently,the risk of the nitrogen gas (N₂) filling the bubble pits oxidizing orcorroding other layers of the substrate, or the like, is very remote,and high storage reliability can be attained.

Moreover, the optical recording medium of the present invention enablessuper-resolution recording and reproduction by means of setting λ/NA to640 nm or less by use of a laser beam having a wavelength of less thanabout 635 nm and an objective lens having a numerical aperture of about0.6-plus. Particularly, a superior characteristic can be acquiredthrough super-resolution recording and super-resolution reproductioninvolving use of a laser beam, whose wavelength is about 405 nm andwhich is to be used with an optical recording medium of next generationtype, and an objective lens having a numerical aperture of about 0.85.Consequently, a recording/reproducing device analogous to arecording/reproducing device for use with an optical recording medium ofnext generation type can be used, and hence cost for developing andmanufacturing the recording/reproducing device can be curtailed.

1. An optical recording medium comprising: a substrate; and a noblemetal nitride layer provided on the substrate.
 2. The optical recordingmedium according to claim 1, further comprising: a first dielectriclayer provided on a light entrance face side of the substrate whenviewed from the noble metal nitride layer; and a second dielectric layerprovided on a side of the substrate opposite the light entrance facethereof when viewed from the noble metal nitride layer.
 3. The opticalrecording medium according to claim 2, further comprising: a lightabsorption layer and a third dielectric layer, which are provided on aside of the substrate opposite the light entrance face thereof whenviewed from the second dielectric layer and arranged in this sequencewhen viewed from the second dielectric layer.
 4. The optical recordingmedium according to claim 3, further comprising: a reflection layerprovided on a side of the substrate opposite the light entrance facethereof when viewed from the third dielectric layer.
 5. The opticalrecording medium according to any one of claims 1 through 4, wherein thenoble metal nitride layer contains platinum nitride (PtNx).
 6. Theoptical recording medium according to any one of claims 2 through 5,further comprising: a light-transmitting layer which is providedopposite to the substrate when viewed from the first dielectric layerand has the light entrance face.
 7. The optical recording mediumaccording to claim 6, wherein a thickness of the substrate ranges from0.6 mm to 2.0 mm; a thickness of the light-transmitting layer rangesfrom 10 μm to 200 μm; a thickness of the noble metal nitride layerranges from 2 nm to 75 nm; a thickness of the second dielectric layerranges from 5 nm to 100 nm; a thickness of the light absorption layerranges from 5 nm to 100 nm; and a thickness of the third dielectriclayer ranges from 10 nm to 140 nm.
 8. A method for manufacturing anoptical recording medium comprising: a first step of forming on asupport substrate, in this sequence, a reflection layer, a thirddielectric layer, a light absorption layer, a second dielectric layer, anoble metal nitride layer, and a first dielectric layer; and a secondstep of forming a light-transmitting layer on the first dielectriclayer.
 9. The method for manufacturing an optical recording mediumaccording to claim 8, wherein processing pertaining to the first step isperformed by means of a vapor phase deposition method, and processingpertaining to the second step is performed by means of a spin coatingmethod.
 10. A data recording method for recording data on the opticalrecording medium defined in any one of claims 1 through 7, to thusrecord data by irradiating a laser beam from the light entrance face,wherein, when a wavelength of the laser beam is taken as λ and anumerical aperture of an objective lens used for focusing the laser beamis taken as NA, a train of record marks, including record marks whoselengths are λ/4NA or less, is recorded by setting λ/NA to 640 nm orless.
 11. A data reproduction method for reproducing data from theoptical recording medium defined in any one of claims 1 through 7, tothus record data by irradiating a laser beam from the light entranceface, wherein, when a wavelength of the laser beam is taken as λ and anumerical aperture of an objective lens used for focusing the laser beamis taken as NA, data are reproduced from a train of record marks,including record marks whose lengths are λ/4NA or less, by setting λ/NAto 640 nm or less.